Chibikart: Finishing Touches and More Testing!

hello internet

I’m actually not sure where the whole “Mariokart” thing that’s going around the various tech websites originated from, but rest assured that Mariokart was not an influencing factor or inspiration for this design. If anything, shouldn’t it have comically large, not comically small wheels to be considered a Mariokart? For something slightly closer to Mariokart, see LOLrioKart, a project of mine from Back In The Day.

Specific offenders: Gizmodo (and Kotaku), Buzzfeed, HuffPo, PCWorld, Daily Mail, Geekologie

Example of interesting coverage with no hackneyed Mariokart jokes: Hackaday, Wired, Redorbit

 

Chibikart, being a relatively quick CAD-to-completion project, is not without its share of random late-night-CADing-induced “This is TOTALLY a great idea!” moments. None of my projects are really complete without one. The first, and most immediately apparent, is this:

Chibikart’s steering linkage is made from two rod ends meeting at right angles. This presented a constraint problem since technically the majority load coming from actuating the steering connecting link (the threaded rod and ball joint) is in torsion. While using a “jam nut” fastening method worked most of the time, if anyone (namely, anyone who wasn’t me and didn’t know of this failure condition) wanged the steering too far, it would twist the black plain rod end. The result was totally changed toe angles and usually the wheels ending up pointed in opposite directions.

And then, for another example, someone running Chibikart into a wall when taking a turn too wide and just straight up shearing off the thing. I was surprised at how soft the steel alloy was. Since I deviated from my usual habit of “buy twice the amount you need” here, I was left with no replacements and also no choice but to quickly re-engineer the thing…

In a move I haven’t actively pulled since the undergrad years of yore (like maybe last year or something), I whipped up two replacements wheel mounting blocks out of solid aluminum billet in about 2 hours. These are essentially identical to the existing blocks except with a .25″ wide “ear” that sticks out the same distance as the rod end linkages did. Much better, more consistent, and it gained me another 2 or 3 degrees of steering travel  due to eliminating the big head of the rod end.

This block is also the proper height for shimming away from the upright arms. Now, none of the components rub on eachother and the steering motion is lighter than ever.

Next order of business was at least getting the rear brake parts installed. Here’s the main body of the thing with the Razor fender mounted. It pivots on a 5mm cap screw which is doubly nutted through the mounting plate. The stock Razor torsion spring keeps it sprung “up”.

It slips onto the existing wheel mounting block like so, and is retained from rotation mostly by bolt clamping pressure but I threw in the set screw locking pin just in case.

And it’s mounted. The brake will not be sticking up this far once it’s ready – I have yet to make the rolley cam thing which will actuate it. In its rest position, the top surface will be approximately horizontal.

Both sides installed. If nothing else, these are pretty neat looking fenders on their own.

Oh, so here’s a picture I forgot about last time: size comparison!

Chibikart was tongue-in-cheek designed to be “exactly the front half of tinykart“. It’s pretty close, I think.

And now, more hoonage.

Chibikart was sent up the de-facto MIT Random Contraption Proving Ground, where it completely defied (again) my expectations. The metric for performance on these tests is minimization of the energy-time product. From start to finish, the watt hours of battery energy consumed and the time taken in seconds is multiplied together. Now, the only physical unit that represents joule seconds is the Planck constant, so it’s essentially just a “score”. However, it’s a very telling score. Divide the energy consumption by the time taken and you have average wattage used in the climb (Joule / second). From the average wattage into the system you can remove estimates of motor losses (such as I^2R loss in the windings) and drivetrain losses to get an idea of the output watts. The game is one of efficiency – doing the most work while wasting the least energy and taking the least time.

Clearly, rider weight matters significantly in such a test since for the most part the You weighs much more than the vehicle.

Anyways, back to Chibikart. I managed a run that was only 62 seconds and 18.6Wh – which is on par with Melonscooter being ridden very fast, but consumed more energy – probably because direct drive magnifies motor imperfections. The total product, 1153 Whs, is actually pretty unique in the range of vehicles that have seen this test so far, and is the second best set of go-kart times (after tinykart, which holds the all-time record so far).

Chibikart pulled 1900W on launch as measured by the wattmeter, and the average draw going up the hill going by the Wh results is approximately 900W, or 225W per motor. Pretty close to the predictions. I didn’t observe any “motor unhappiness”, but that was likely due to the outside air temperature being something like 38 degrees at the time.

Here’s a Helpful Infographic made by the master of making helpful diagrams and infographics, Shane. This really needs to be zoomed in to be appreciated:

And the “initial hoonage” video:

Chibikart: The Race To Completion

I’m happy to say that Chibikart has exceed alot of expectations.

Does this mean I finished it, finally? Yes. The build has taken in total about 3 weeks from announce to first ride test, which is rather short for one of my usually long and drawn out projects, but I was helped greatly by previously-made parts and an easy-to-assemble frame…..and not having to build a controller for it. Here’s how the remaining parts went down. The obligatory hoonage video is at the bottom.

First order of business was making the rear wheel anchor blocks. Unlike the front wheel which has tapped threads in the block (due to the need to pass the steering kingpin), the rear wheel is a fully bolt-through configuration, with the socket head 1/4″-20 bolt sitting in a counterbored hole.

With that part finished, I could finally put Chibikart on four wheels…

There was much push-riding involved. The steering was found to be very nimble and light , probably because of the very low drag wheels and high lever ratio. This thing also rolls. Because the hub motors have so little cogging and the wheel is moderately hard, it’s almost as good as a ball bearing caster wheel. I spent a few minutes finding “low spots” in the IDC lab space hallways, because Chibikart would actually start rolling towards them if pointed in the right direction.

I’m also glad to see that the wheel mounting scheme does not deflect much under even large white guys riding it (myself being relatively small and vaguely Asian). Granted, this is all on a smooth linoleum floor, and the real structural test will be when it goes outside.

Now, to do something about the baseplate upon which all the electronics will be mounted. It’s waterjet-cut from a 1/8″ polycarbonate sheet. I put in some slots for nylon/velcro strapping in order to hold the battery, and some holes to mount the Jasontrollers, but otherwise planned on freehanding all the necessary distribution electronics and other parts. I also cut out some test parts for the planned mechanical brake, but those are not yet installed.

So here’s the big generically orange block that was in the CAD model. It’s a 10S2P A123 battery module using 32113-type automotive cells. The total capacity of the pack is about 300Wh. That means Chibikart has an asston of battery for such a small vehicle. Now, while I do have giant Turnigy lithium sticks left over from the giant quadrotor of doom which are smaller by volume, this random surplus pack fit the frame so perfectly that I had to use it. It’s also more enclosed and has an internal BMS.

I didn’t take many pictures of the wiring process this time, because it’s very simple. In the upper right is a one-in-four-out terminal block which conveniently splits the battery input to exactly the number of power connections necessary. The throttle signal from the pedal gets split at a terminal block into the four Jasontrollers, each of which is otherwise only hooked up to power, ground, and motor. That’s it – 8 wires. That’s what I like about these things – they’re so bone simple yet effective.

Granted they also come with about 8,000 other wires which perform random generic electric bike functions (like pedal sense assist, cruise control, just not regnerative braking for some reason), and for all intents and purposes they don’t exist for this project.

At the pictured stage, I tried Chibikart in 2-wheel-drive mode with only the rear motors connected. As expected, starting from standstill is a little challenging because the motors have so little torque relative to my inertia. However, any movement at all is enough to cause the Jasontrollers to lock in and begin applying drive torque – even wiggling back and forth in the seat. They have quite an effective startup routine for what they are and how much they cost.

I expected that with 4 motors, it will either be better (all the motors contribute to starting torque) or totally useless (the controllers “park” the motors at the start in different directions, they fight eachother, and I have to perform an in-situ hip thrust to begin moving). Afterwards, I sacrificed some spare IEC power cords for their 18-gauge, 3-conductor hearts and extended wiring runs out to the front motors. Turns out, either situation can happen depending on where the motors stop – go figure.

I measured the current draw under acceleration at the motor for a totally stock (unmodified and unopened) 350W Jasontroller, and it was about 22 amps. Running at 32v, that means a maximum power throughput of 600W… Since the 36v native design can in fact run unmodified up to ~44v, it means that these controllers are a rare instance of some shady eBay Chinese product being underrated.

Here’s the “press shot” of the whole thing. I haven’t weighed it in yet, but it “feels” about 40 pounds. I must say I’m very pleased with the full 4-motor performance of the thing – I had expected that it would be on par with the skate motors (or at least, their very power-limited incarnation in the skates), but for some reason these are way peppier. This is probably because of their higher ampere limit (~20 amps per motor) and sliiiightly better efficiency. It remains to be seen how Chibikart handles an entire day of running outside, like at Swapfest as MIT vehicles tend to be debuted, instead of inside the cool, smooth, air conditioned hallways. Also, that lawn tractor seat is actually quite comfy.

And as promised, the video!

The short story:

  • Frame size: 34″ x 18″
  • Motors: Custom-wound and packaged direct drive hub motor, 300W peak each*
  • Wheels: 100mm 87A skate wheels
  • Battery: 32v 9Ah lithium iron phosphate pack
  • Top speed, theoretical: 26mph (voltage & motor RPM/V & wheel diameter)
  • Top speed, realistic: 21mph**
  • Actual top speed: To be determined?!
*30 second “peak” rating at 20 amperes
**Factoring in conservative estimates for air drag, and motor resistive losses at-speed, smooth and level ground assumed.